Objective Data - Discharge Testing

Part 1 revealed that not all batteries are equal, despite what the label reads. Some "feel" better than others. That isn't the most helpful way to review or compare batteries. This one "feels" better than the other. How much better? How will it "feel" when you change props, fly faster, fly longer? To really understand what performance to expect (or not expect) we need objective data.

Testing Methodology

This certainly won't be as complex or accurate as engineers would want, however it is reproducible by others. It may even be more accurate and sophisticated than what some of the companies selling batteries use (I suspect some don't do any testing).

The rig I built uses a GT Power Watt meter, halogen light bulbs, a video camera, multimeter, IR Thermometer, and batteries. I will post a how-to build log on how you can build your own. Total cost was about $40. The Watt meter connects to the battery. A multimeter is connected to the balance port of the battery to monitor the voltage there. Why? This measures the voltage at the pack and not on the circuit with the load. It is more accurate this way.

To produce a current draw I used halogen light bulbs. I got 50W, 10W and 5W 12V bulbs. This will allow me to build modules of different wattage so I can test a variety of loads. For these tests I used 6 50W bulbs and 2 10W bulbs. This results in an average current of 27 Amps. Not a perfect setup, but when consistently used it will result in meaningful and comparable data. I have done repeats of the test and the results are nearly identical.

The ambient and pack temperature do have a measurable and significant impact on the results. However as long as all trials are consistent, the data is relative. Test temperature is between 75F and 77F.

Video recording of this is started (to record data points and provide a reference for review).

Battery pack initial temperature is taken.

27A Load is connected.

Battery pack temperature is measured throughout the entire process. Temperature is taken at the hottest point (on the side near where the wires exit).

Load is removed when the pack reaches 80% discharge (unless I'm doing a deeper discharge to find the 'knee' and capacity).

Battery is moved to a cooling fan. Recording stopped.

Data input into spreadsheet.

Pretty graph published.

After a few trials I found the results to be very reproducible. I'm confident in this process. My background includes working in a lab on projects subject to extreme legal scrutiny i.e. national advertising campaigns for billion dollar brands.

The data recorded below is:

Internal Resistance (IR) - The higher the IR the more heat the battery generates. This is a good benchmark of poor performance. Lower numbers are better. For these 1300 mAh cells 5mOhms is very good, under 10mOhms is OK, over 10mOhms is poor. A "Pack IR" is the total of the cells plus the power leads and connector.

LVC (Low Voltage Cutoff) - How many mAh were used before it hit 10.8V. I looked at the data to see when the voltage dropped below 3.6V per cell. This is a good indicator of when your LVC or alarm may trigger. It also indicates when performance drops will be noticeable. Higher is better.

Average Voltage - the average voltage across the test. A simple way to measure start to finish performance. Higher is better.

Average Voltage for first 50% - many only use 50-60% of their packs capacity. Why? They can fly and have packs that are at storage voltage, no need to apply a storage voltage. This helps prolong their life because you don't discharge them too deep or leave them stored below 3.8V/cell. This is also the most critical time for performance. Higher is better.

Final Voltage - what the voltage was after 150 seconds under 27A load. NOT the resting voltage the pack returned to. Higher is better.

Final Temperature - Temperature of the pack after 150 seconds under 27A load. Heat can damage your battery. Lower is better.

Weight - pack weight in grams

The Results

I'll let the data speak for itself. I'll only post the numbers and objective data.

Turnigy Graphene 1300mAh

Discharge Graph

The following graph shows the full discharge curve of each pack at 27A. Its a bit crowded, but you should be able to see how the various packs perform.

Comments and observations

The data was pretty consistent across several tests and repeats. If the data was different it was a measure of hundredths of a volt. The data also correlated very well with actual flight tests. The SMC, Bonka, Lumenier 60C, Graphene, and VCANZ all performed very similarly.

The Lumenier 35C however shows it is clearly not of the same caliber. This was observed using two different packs and several repeated tests. Most notably, the 35C Lumenier was very hot. Too hot to hold comfortably. This was in an open environment. Had I simulated a model like a foam plane, the temp would be much higher. Lipo manufacturers warn of high temps. 160-170F INTERNAL is where they draw the line (about 135 external). 157F at just 20C means this pack would likely not survive 35C. That means a crashed model or worse.

A surprise was the Bonka batteries. At just 45C they matched the Lumenier. Bonka also sells higher C rated packs, can they do better? (recent testings shows no, the 45C performs as good as their 65C).

The real shocker? The SMC "37A True Spec". If you were to calculate a C rating it would be just 28.5C! Yet it beat the packs with more than double the C rating. That says two things: C rating is bunk and SMC makes great packs!

You can't go wrong with any of the better packs. Want the best value? The Bonka and SMC seem to take the lead there. They offer great peformance and the to your door cost makes them very affordable. The Graphene is a very good option. It is larger and heavier, but the voltage difference is pretty clear and sure to give you a bit more punch. The extra weight and bulk may throw off your balance/tuning enough to negatively affect flight characteristics. All things considered the best pack of the bunch may be the SMC. The best performance, low weight, and from a brand with a proven history of reliable Lipo packs.